Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 190441

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1041 PM CST Thu Jan 18 2018

120 PM CST

Through Friday...

The clipper system to our north will quickly exit the region
tonight. Southwest winds have ushered in some more seasonal
temperatures getting close to the freezing mark. High pressure
aloft will shift across the Great Lakes tonight. At the surface
high pressure will to the southeast USA, while a second low will
dig into south central Canada. This will maintain breezy southwest
winds through tonight and Friday. Expect lows to hold near or even
above yesterday`s highs, with lows tonight mostly in the 20s.
Continued warm advection on Friday will steer high temperatures up
near 40. We will have some higher clouds due to the low passing by
to the north and from a system well out in the west.



326 PM CST

Friday night through Thursday...

Warm trend continues at the start of the period, with dry
conditions also persisting. Initial zonal flow will transition to
more ridging as upper level low/trough moves through the central
CONUS this weekend. In this pattern, WAA will be the trend with
lows Friday night in the 20s rising to highs around 40 on
Saturday. However, more pronounced moist advection will be delayed
some, with dewpoints not really rising until late Saturday into
Sunday morning. This will result in increasing low clouds and fog,
with drizzle becoming more likely Sunday night. Similar concerns
with the low chance of freezing drizzle continues, and so have
added mention to the forecast primarily for far northern Illinois.
At this time, it does not appear to be overly concerning but
enough to warrant mention.

Fog concerns really increase into Sunday, with the arrival of 40+
dewpoint air. Increasing forcing will help to continue precip
chances throughout the day Sunday, with light rain becoming more
likely. However, strongest forcing associated with the now closed
low and surface low doesn`t arrive until late Sunday night into
Monday. This should help a more widespread precip shield to spread
across the region, with a period of steady rain likely during
this time. Guidance still indicating some elevated instability
through this period, with rumbles of thunder still a possibility.
Will continue to monitor flood issues/concerns, with the current
snow pack and this possible rainfall, especially if there ends up
being more convective type precip. Cold returns on the backside of
this system later in the day Monday, with any lingering precip
likely transitioning to snow.



For the 06Z TAFs...

Conditions will continue to be fairly quiet with VFR conditions
over the next 24 to 30 hours. Southwest winds (200-220 degree
direction) will increase by the late morning or early afternoon
and become gusty. Expect gusts generally in the 20-25 kt range
through Friday evening, with winds and gusts then diminishing
overnight. The low level jet will ramp up Friday evening, causing
southwest winds at FL ~020 to increase to about 50 kt. If wind
gusts are less frequent than expected, LLWS may need to be added
to the TAF.



326 PM CST

Southwest winds to 30 kt still in place this afternoon as low
pressure pushes east through southern Ontario. Expect these winds
to continue into this evening, but with some weakening expected as
the low weakens. This trend should also continue for the
nearshore waters, but with winds remaining elevated, have extended
the small craft advisory into this evening. A new low will move
towards the Great Lakes region out of Canada on Friday, with winds
expected to increase over the entire lake. With gales appearing
likely later in the day Friday into Friday evening, have replaced
the gale watch with a gale warning. Speeds increase over the
nearshore waters as well, but have lower confidence for the
potential for gales during this time. Have maintained winds to 30
kt but did include mention of gale force gusts. Will need to
monitor the potential for more prevailing gales over the nearshore
waters Friday afternoon into Friday evening.



LM...Gale Warning...LMZ777-LMZ779...2 PM Friday to 3 AM Saturday.

     Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...3 PM Friday to 3 AM Saturday.




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